In the electric power industry, direct-current (DC) distribution systems, DC energy sources such as photovoltaic (PV) panels and fuel cells, and DC-based energy storage systems are of increasing interest. In particular, considerable research is directed to the development of practical energy storage systems based on the use of batteries, superconductive energy storage systems, flow batteries, super-capacitors, and the like.
Frequently, a DC energy source or DC loads need to be connected to an alternating-current (AC) grid, AC source, or AC load. This is conventionally done with a grid-tie converter that converts AC into DC and/or vice-versa. Hereinafter, such a converter is referred to as an AC-DC converter or, more simply, a power converter, even in the event that the converter allows for bidirectional power flow. Thus, the term AC-DC converter as used herein should not be understood to be limited to converters in which the power flow is exclusively from the AC side to the DC side. In some cases, the power flow is only one way, e.g., in the case of a solar panel system supplying energy to an AC grid. In other cases, however, such as with the energy storage systems mentioned above, the grid-tie converter must handle power flow in both directions, e.g., supporting power flow from the AC side to the DC side while charging a battery-based system, and in the reverse direction when drawing energy from the batteries.
Accordingly, AC-DC converters that can handle bi-directional power flow and that have high-efficiency, low distortion, and improved cost and size are needed.